1. Field of the Invention
This invention relates to a method for preventing the deposition of scale in subterranean formations or in well fluid-gathering metal equipment exposed to high temperature aqueous media. More particularly the invention relates to such a method for preventing the deposition of scale onto formation rock or metal surfaces from geothermal fluids containing calcium and other scale-forming ions during production of the geothermal fluids from a subterranean formation via a well and the subsequent utilization of the produced fluid.
2. Description of the Prior Art
One source of recoverable natural energy is geothermal energy stored in hot subterranean formations. One way of utilizing this geothermal energy involves drilling one or more wells into a subterranean formation which may contain either a geothermal fluid or hot dry rocks. If it contains a geothermal fluid, the fluid may be produced via a well. If it contains only hot dry rocks, a relatively low temperature heat exchange fluid is passed through the formation and recovered via a well after it has been heated by the rocks. In either instance, the process involves, in part, the production of geothermal fluids from the formation to the surface via a well and subsequent handling of the fluid to utilize the geothermal energy. Such utilization may involve electric power generation by using the geothermal energy to drive a turbine, passing the fluid through a helical rotary screw expander power system, use in a binary power cycle with a working fluid such as isobutane in a regenerative heat exchanger, or direct utilization for its heat, water or minerals content, for space heating or process heating.
Within the formation, geothermal fluids are at high temperatures and pressures. Under such conditions, geothermal liquids characteristically contain considerable amounts of various dissolved salts. The production and subsequent utilization of these fluids involve passing them through the formation to a well and then through metal conduits and other fluid handling equipment. During this time, the fluid cools and the pressure is reduced. When this change in condition occurs, the dissolved salts tend to precipitate, adhere to the solid rock or metal surfaces with which they come in contact, and build up in thickness so as to eventually cause plugging of the formation and/or the fluid handling equipment and/or a reduction in the heat transfer properties of the system.
Under other less severe scaling conditions, such as with aqueous solutions, for example brines, flowing through fluid handling equipment at temperatures up to about 200.degree.F., it has been proposed to add various materials to retard scale formation. Among the scale inhibitors suggested have been a wide variety of phosphonic acid derivatives. For example, U.S. Pat. No. 3,336,221 issued Aug. 15, 1967to Paul H. Ralston describes a method for inhibiting the formation of scale from a system of water or brine containing alkaline earth metal cations and various anions by adding to the system a methylene phosphonic acid or alkali metal or ammonium salt thereof bonded to a nitrogen atom, for example, N,N,N',N'-tetrakis(methylene phosphonic acid) ethylene diamine or a salt thereof. U.S. Pat. No. 3,867,286 issued Feb. 18, 1975 to Patrick M. Quinlan shows polyquaternary ammonium methylene phosphonates as scale inhibitors for water. Other references teach preventing scale formation from commercial waters by adding thereto a mixture of one or more phosphonic acid derivatives and another compound. British Pat. No. 1,361,857 published July 31, 1974 discloses an acrylic or methacrylic polymer as the other compound in the inhibitor mixture. British Pat. No. 1,330.596 published Sep. 19, 1973 utilizes 1,2,3,4,5-penta-carboxylic acid in the inhibitor mixture. Other references teaching the use of various phosphonic acid derivatives as corrosion inhibitors in aqueous systems are: U.S. Pat. No. 3,868,216 issued Feb. 25, 1975 to William Robert Hollingshad, U.S. Pat. No. 3,880,584 issued Apr. 29, 1975 to Derek Redmore, British Pat. No. 1,386,746 published Mar. 12, 1975 and British Pat. No. 1,386,718 published Mar. 12, 1975.
It was found that the scaling problems encountered in handling geothermal liquids are more severe and more difficult to control than in handling water or brine at lower temperatures. Most inhibitors which are effective in controlling scale formation from water and brine are not effective in the higher temperature geothermal liquids.
Accordingly, a principal object of this invention is to provide a method for inhibiting the formation of scale from geothermal fluids during their production from subterranean formations and subsequent handling.
Another object of the invention is to provide such a method for inhibiting the formation of a calcium-containing scale onto the metal surfaces of geothermal fluid production and fluid handling equipment.
A further object of the invention is to provide such a method for inhibiting the formation of scale from liquid-containing geothermal fluids having temperatures above about 400.degree.F.
It is a still further object of the invention to provide such a method for inhibiting the deposition of scale from high temperature wet steam.
Other objects, advantages and features will become apparent from the following description and appended claims.